1. Field of the Invention
The present invention relates to ferroelectric liquid crystal displays (LCDs), and more particularly to a ferroelectric LCD and a method of driving the same according to an impulse-type method wherein a voltage holding ratio (VHR) of liquid crystal cells is reduced.
2. Description of the Related Art
Generally, LCDs display pictures by applying electric fields to a layer of liquid crystal material in response to an applied video signal, wherein the applied electric field controls the orientation liquid crystal molecules within the layer of liquid crystal material and therefore the light transmittance characteristics of the liquid crystal material.
FIG. 1 illustrates a related art active matrix LCD (AM-LCD).
Referring to FIG. 1, related art AM-LCDs generally include an LCD panel formed of lower and upper substrates. The lower substrate (i.e., the TFT array substrate) supports a plurality of data lines D1 to Dm and a plurality of gate lines G1 to Gn arranged to cross each other. Liquid crystal cells (Clc) are defined by, and thin film transistors (TFTs) are formed at, the crossings of the gate and data lines, wherein the TFTs are provided for driving corresponding ones of liquid crystal cells. A linear polarization filter having a first polarization axis is mounted onto the rear surface of the TFT array substrate. A protecting film is formed over the entire front surface of the TFT array substrate in order to cover the TFTs and the gate and data lines. An alignment film is formed over the entire surface of the protecting film. Alignment grooves are formed in the alignment film for aligning subsequently provided liquid crystal material. The front surface of the upper substrate (i.e., the color filter substrate) supports a black matrix, a color filter, a common electrode, and an alignment film. The rear surface of the color filter substrate supports a polarization plate. The upper and lower substrates are bonded together by a sealant and liquid crystal material is injected between the bonded substrates. Storage capacitors (Cst) are connected to corresponding ones of the liquid crystal cells (Clc) and allow each liquid crystal cell (Clc) to maintain an applied data voltage.
Storage capacitors (Cst) of each liquid crystal cell (Clc) include an electrode connected to one portion of a corresponding pixel electrode and another electrode connected to a preceding one of the gate line G1 to Gn-1. Gate electrodes of each TFT are connected to the gate lines G1 to Gn that transmit scan pulses. Source electrodes of each TFT are connected to the data lines D1 to Dm that transmit data voltages. Drain electrodes of each TFT are connected to respective ones of the pixel electrodes within the liquid crystal cell (Clc).
The black matrix is formed in boundary regions between adjacent liquid crystal cells (Clc) and in TFT regions of each liquid crystal cell (Clc) to absorb incident light and prevent light transmitted though portions of the liquid crystal material between adjacent liquid crystal cells, having undesirable optical properties, from being displayed by the LCD. The color filter selectively transmits light having predetermined wavelength ranges corresponding to red, green, and blue colors.
Referring to FIG. 2, LCD display pictures by sequentially applying scan pulses SP1 to SPn to the gate lines G1 to Gn. Horizontal lines of liquid crystal cells connected to gate lines having scan pulses applied to them (e.g., having selected gate lines) transmit light in accordance with an analog data voltage applied by the data lines D1 to Dm, wherein the analog data voltage corresponds to a received video signal.
Referring to FIG. 3, scan pulses SP1 to SPn are generated using a shift register having stages ST1 to STn electrically connected to corresponding ones of the gate lines G1 to Gn, respectively. The first stage (ST1) of the shift register generates a scan pulse (SP1) in synchrony with a gate start pulse (GSP) outputted from a timing controller (not shown). The second to nth stages ST2 to STn sequentially generate scan pulses SP2 to SPn, respectively, in response to preceding ones of the scan pulses SP1 to SPn-1, respectively, supplied from the preceding stage. A gate shift clock (GSC) signal and a gate output enable (GOE) signal control the output timing of the scan pulses.
In related art LCDs, during one frame period of the LCD panel (i.e., 16.67 ms, as defined by the National Television System Committee (NTSC)), the scan pulses SP1 to SPn are sequentially applied to the first gate line to the nth gate line G1 to Gn in response to the generated gate start pulse (GSP). Each scan pulse SP1 to SPn is applied to a corresponding gate line G1 to Gn once during one frame period of the LCD panel.
Generally, the liquid crystal material within LCDs exhibits an intermediate material phase between solid and liquid phases wherein liquid crystal molecules exhibit both fluidity and elasticity. Currently, the most common type of liquid crystal material used in LCDs include twisted nematic mode (TN mode) liquid crystal material.
Referring to FIG. 4, a high logic voltage (Vgh) of each scan pulse (SP) is greater than a threshold voltage of a TFT and forms a conductive channel between the source/drain electrodes of the TFT such that liquid crystal cells of TN mode LCDs become charged with a voltage (Vlc). Accordingly, the liquid crystal cell substantially maintains the charged voltage during the entire frame period of the LCD panel. A low logic voltage (Vgl) of the scan pulse (SP) is lower than the threshold voltage of the TFT. A voltage holding ratio (VHR) of each liquid crystal cell is the difference between a peak data voltage (Vdata) of the data voltage applied to the liquid crystal cell and an average voltage (Vavrg) of voltage actually charged by the liquid crystal cell when the peak data voltage (Vdata) is applied. The larger the voltage holding ratio (VHR), the larger the difference between the data voltage applied to the liquid crystal cell and the voltage actually charged by the liquid crystal cell. Accordingly, as the voltage holding ratio increases, the ability to control the brightness of the liquid crystal cell decreases.
Undesirably, TN mode liquid crystal material has a relatively low response speed and a relatively narrow viewing angle. To overcome the aforementioned problems, TN mode liquid crystal material can be replaced by ferroelectric liquid crystal (FLC) material having a response speed and viewing angle generally greater than that of TN mode liquid crystal material. FLC material exhibits a lamellar structure, wherein each layer of FLC material has the same electric and magnetic properties. Accordingly, when FLC material is driven, molecules of FLC material within the same layer spontaneously rotate (i.e., polarize) along a virtual cone in response to an applied electric field. In the absence of an applied electric field, molecules within the FLC material spontaneously polarize to an original alignment orientation. Accordingly, when external electric fields are applied to the FLC material, molecules within the FLC material rotate rapidly by interaction of an external field and exhibit spontaneous polarization. The response speed of FLC material is typically between a hundred or a thousand times faster than other modes of liquid crystal material. Further, FLC material has an inherent in-plane-switching property and can therefore be used to provide LCDs with wide viewing angles without special electrode structures or compensation films. Further, FLC material has an inherent in-plane-switching property and can therefore be used to provide LCDs with wide viewing angles without special electrode structures or compensation films. Depending on its behavior in the presence of applied electric fields, FLC material may be classified as V-Switching or Half V-Switching Modes.
V-Switching Mode FLC material exhibits the following thermodynamic phase transformations upon decreasing temperature: isotropic→smectic A phase (SA)→smectic X phase (Sm X*)→crystalline. At the isotropic phase, molecules within the FLC material are oriented and distributed substantially isotropically (e.g., randomly). At the smectic A phase (SA) phase, molecules within the FLC material are divided into symmetrically arranged layers of vertically arranged molecules. At the smectic X phase (Sm X*) phase, molecules within the FLC material are arranged according to an intermediate order between smectic A and crystalline phases.
FIG. 5 illustrates a graph of transmissivity of incident light versus voltage applied to a V-Switching Mode ferroelectric liquid crystal cell.
Referring to FIG. 5, the transmissivity of light incident to a V-Switching Mode FLC cell exhibiting the smectic X phase (Sm X*) is dependent upon the polarity of an applied driving data voltage (e.g., +V and −V). Accordingly, the arrangement of liquid crystal molecules within V-Switching Mode FLC material may be affected by the applied external voltage. V-Switching Mode FLC material beneficially has high response speed and wide viewing angle characteristics but disadvantageously requires a large amount of power in order to be driven because a capacitance value of the V-Switching Mode FLC material is relatively large. Therefore, a capacitance value of a storage capacitor used to maintain applied data voltages are also be large. Accordingly, if V-Switching Mode FLC material is used LCDs, an aperture ratio of the LCD becomes low since the power consumption of LCD and an electrode area of a sub-capacitor increases.
Half V-Switching Mode FLC material beneficially has a high response speed and wide viewing angle characteristics and further has a relatively low capacitance value. Therefore, Half V-Switching Mode FLC material is often used to display moving pictures.
FIG. 6 illustrates phase transformations of Half V-Switching Mode ferroelectric liquid crystal material.
Referring to FIG. 6, upon decreasing temperature below the phase transformation temperature (Tni), Half V-Switching Mode FLC material exhibits a phase transformation from the isotropic to the nematic phase (N*), below phase transformation temperature (Tsn), the Half V-Switching Mode FLC material exhibits a phase transformation from the nematic phase (N*) to the smectic C phase (Sm C*), and below phase transformation temperature (Tcs) the Half V-Switching Mode FLC material exhibits a phase transformation from the smectic C phase to the crystalline phase. Therefore, as the temperature decreases, Half V-Switching Mode FLC material exhibits the following thermodynamic phase transformations: isotropic→nematic (N*)→smectic C phase (Sm C*)→crystalline.
FIG. 7 illustrates the fabrication of a liquid crystal cell including Half V-Switching Mode FLC material.
Referring to FIG. 7, Half V-Switching Mode FLC material is typically injected into a liquid crystal cell at a temperature above Tni. Accordingly, upon being injected into the liquid crystal cell, molecules within the FLC material are oriented and distributed substantially isotropically (e.g., randomly). Upon lowering the temperature of the FLC material below Tni, molecules within the FLC material become aligned substantially parallel to each other along a direction dictated by the rubbing direction of an orientation layer and the FLC material exhibits the nematic phase (N*). If the temperature of the FLC material is further lowered the temperature below Tsn in the presence of an electric field, the FLC material exhibits the smectic phase (C*) and the liquid crystal molecules spontaneously polarize along the direction of the applied electric field to exhibit a monostable state, wherein liquid crystal molecules uniformly assume one of two possible molecular arrangements. If, on the other hand, the temperature of the FLC material is lowered below Tsn in the absence of the applied electric field, the liquid crystal molecules become separated into layers to exhibit a bistable state, wherein liquid crystal molecules within each layer uniformly assume one of the two possible molecular arrangements. Further, the distribution of the molecular arrangements within the layers is substantially random. In view of the above, it generally more difficult to uniformly control of the FLC material exhibiting the bistable than to uniformly control of the FLC material exhibiting the monostable state. Accordingly, the Half V-Mode FLC cells are generally fabricated to exhibit the monostable state by cooling the FLC material below Tsn in the presence of an electric filed generated by applying a small direct current (DC) voltage to the LCD panel.
Referring still to FIG. 7, the symbol “” describes the direction of the applied electric field as extending out of the plane of the illustration. Therefore the spontaneous polarization direction of the FLC material also extends out of the plane of the illustration. Accordingly, electrodes used to generate the applied electric field are formed on upper and lower plates of the liquid crystal cell, extending out of the plane of the illustration. Further, the orientation layer described above is formed on the upper and lower plates.
FIGS. 8A and 8B illustrate the dependence of light transmissivity on a voltage applied to a Half V-Switching Mode FLC cell.
Referring to FIG. 8A, Half V-Switching Mode FLC cells containing FLC material aligned in the presence of an applied electric field generated by a voltage having a negative polarity (−V) (i.e., fabricated in the presence of an electric field having a negative polarity), transmit light in the presence of an applied voltage having a positive polarity (+V) by rotating a polarization axis of the light by 90°. The light transmissivity of the Half V-Switching Mode FLC cell increases proportionally to the intensity of an applied positive electric field generated by the positive voltage (+V). Further, the light transmissivity of the Half V-Switching Mode FLC cell attains a maximum value when the intensity of the applied positive electric field is greater than a fixed threshold value of the FLC material. In the presence of an applied voltage having a negative voltage (−V), the Half V-Switching Mode FLC cell does not rotate the polarization axis of the light. Accordingly, in the presence of an applied voltage having a negative polarity, the Half V-Switching Mode FLC cell transmits substantially no light (i.e., the Half V-Switching Mode FLC cell intercepts the light).
Referring to FIG. 8B, Half V-Switching Mode FLC cells containing FLC material aligned in the presence of an applied electric field generated by a voltage having a positive polarity (+V) (i.e., fabricated in the presence of an electric field having a positive polarity), transmit light in the presence of an applied voltage having a negative polarity (−V). Further, in the presence of an applied voltage having a positive polarity (+V), the Half V-Switching Mode FLC cell does not rotate the polarization axis of the light. Accordingly, in the presence of an applied voltage having a positive polarity, the Half V-Switching Mode FLC cell intercepts the light.
FIGS. 9A and 9B illustrate the orientation directions of Half V-Switching Mode FLC material in the presence of applied electric fields used to fabricate the liquid crystal cell and applied electric fields used to drive the liquid crystal cell, respectively.
Referring to FIG. 9A, when the Half V-Switching Mode FLC cell is fabricated in the presence of an externally applied electric field generated by a voltage having a negative polarity, the spontaneous polarization direction (Ps) of FLC material becomes uniformly aligned to the direction of the externally applied electric field having the negative polarity (E(−)). Referring to FIG. 9B, if, during a subsequent driving of the LCD panel, an electric field having a positive polarity (e.g., an electric field generated by applying a voltage having a positive polarity to the LCD panel) (E(+)) is applied to the fabricated Half V-Switching Mode FLC cell, the FLC material spontaneously polarizes along a spontaneous polarization direction (Ps) uniformly aligned with the direction of the applied electric field having the positive polarity. Accordingly, a polarization state of light incident to a lower plate of the LCD panel may be rotated to substantially align with the polarization direction of an upper polarizer on an upper plate via the FLC material, having the spontaneous polarization direction (Ps) uniformly aligned with the externally applied electric field having the positive polarity, and the incident light is transmitted through the upper plate. If, however, during driving of the LCD panel, the applied external electric field is generated by an applied voltage having a negative polarity (and thus itself has a negative polarity (E(−)), or if, during driving, no electric field is applied, the FLC material remains uniformly aligned along its initial spontaneous polarization direction (Ps) (characterized by the applied electric field having the negative polarity) and the incident light beam is not transmitted through the upper plate (i.e., the light is intercepted by the liquid crystal cell).
Referring to FIG. 10, the brightness of the Half V-Switching Mode FLC cell described above is relatively low and cannot precisely express images consistent with received video signals because the average voltage (Vavrg) charged within each liquid crystal cell is low and the voltage holding ratio (VHR) is excessively large. More specifically, the Half V-Switching Mode FLC cell immediately discharges the charged voltage after the peak data voltage (Vdata) is charged when the scan pulse (SP) attains the high logic voltage (Vgh). Accordingly, the liquid crystal cell cannot effectively express gray scale values of expressed images consistent with the received video signal and the brightness becomes much reduced compared to the brightness required by the received video signal when the voltage of the Half V-Switching Mode FLC cell drops abruptly as described above because the average voltage (Vavrg) of the liquid crystal cell becomes much smaller than the peak voltage of the data voltage (Vdata).
If, during fabrication, the entire LCD panel is uniformly aligned under an applied electric field having a single polarity (e.g., a positive polarity (+) or a negative polarity (−)), defects may be generated within the fabricated Half V-Switching Mode FLC cell when the LCD panel is driven according to an inversion driving method. Such defects may be manifested by the lowering of a brightness of displayed pictures and flickering (e.g., blinking) of the displayed pictures. Such flickering may be reduced by employing inversion driving methods to drive LCD panels, wherein the inversion driving methods also prevent a degradation of liquid crystal material within the LCD panel by inverting the polarity of applied data voltages between predetermined periods of the LCD panel. For example, a frame inversion driving method inverts the polarity of data voltages applied between successive frame periods at a frequency of 60 Hz, in correspondence with the frame period of 16.7 ms. A line inversion driving method inverts the polarity of data voltages between successive frame periods and horizontal lines. A column inversion driving method inverts the polarity of data voltages between successive frame periods and vertical lines. Lastly, a dot inversion driving method inverts the polarity of data voltages between successive frame periods, horizontal lines, and vertical lines, as shown in FIGS. 11A and 11B. Because the polarity of data voltages can be inverted between successive frame periods, horizontal lines, and vertical lines, the dot inversion driving method is most commonly used within LCDs to minimize flickering.
Further, LCDs including the aforementioned Half V-Switching Mode FLC cells, fabricated in the presence of a uniformly applied electric field having a negative polarity and arranged in a matrix pattern, may be driven according to the dot inversion driving method. Accordingly, and with reference to FIGS. 11A and 11B, horizontally and vertically adjacent ones of Half V-Switching Mode FLC cells arranged within an LCD panel may alternately transmit and intercept light because each Half V-Switching Mode FLC cells can only transmit light in the presence of an applied electric field having a positive polarity. For example, odd ones of the FLC cells arranged within odd horizontal lines of liquid crystal cells and even ones of the FLC cells arranged within even horizontal lines of liquid crystal cells transmit light in response to an electric field having a positive polarity (+) applied during odd frames (see FIG. 12A) and intercept light in response to an electric field having a negative polarity (−) applied during even frames (see FIG. 12B). Moreover, even ones of the FLC cells arranged within odd horizontal lines of liquid crystal cells and odd ones of the FLC cells arranged within even horizontal lines of liquid crystal cells transmit light in response to an electric field having a positive polarity (+) applied during even frames (see FIG. 12B) and intercept light in response to an electric field having a negative polarity (−) applied during odd frames (see FIG. 12A).
LCDs including a plurality of Half V-Switching Mode FLC cells, fabricated in the presence of a uniformly applied electric field having a negative polarity and arranged in a matrix pattern, may be driven according to the dot inversion driving method. Accordingly, and with reference to FIGS. 12A and 12B, horizontally and vertically adjacent ones of Half V-Switching Mode FLC cells arranged within an LCD panel may alternately transmit and intercept light because each Half V-Switching Mode FLC cells can only transmit light in the presence of an applied electric field having a positive polarity. For example, odd ones of the FLC cells arranged within odd horizontal lines of liquid crystal cells and even ones of the FLC cells arranged within even horizontal lines of liquid crystal cells transmit light in response to an electric field having a positive polarity (+) applied during odd frames (see FIG. 12A) and intercept light in response to an electric field having a negative polarity (−) applied during even frames (see FIG. 12B). Moreover, even ones of the FLC cells arranged within odd horizontal lines of liquid crystal cells and odd ones of the FLC cells arranged within even horizontal lines of liquid crystal cells transmit light in response to an electric field having a positive polarity (+) applied during even frames (see FIG. 12B) and intercept light in response to an electric field having a negative polarity (−) applied during odd frames (see FIG. 12A).
Referring still to FIG. 12A and FIG. 12B, reference numerals ‘P1’ and ‘P2’ indicate the polarization axes of polarization plates arranged on upper and lower substrates of the LCD panel, respectively. The polarization axis of each polarization plate determines polarization characteristics of light it will transmit. As shown in the Figures, the polarization axes of the upper and lower polarization plates are substantially perpendicular to each other. Within liquid crystal cells transmitting light, light having a polarization direction parallel to P1 (or P2) is transmitted by an incident polarization plate, through the FLC material, and is subsequently transmitted by a display polarization plate where the light transmitted by the display polarization plate has a polarization direction parallel to P2 (or P1). Within the liquid crystal cells intercepting light, light having a polarization direction parallel to P1 (or P2) is transmitted by an incident polarization plate is not transmitted to the display polarization plate having the polarization axis of P2 (or P1).
FIG. 13 illustrates a graph of a data voltage charged to a Half V-Switching Mode FLC cell within an LCD panel and the corresponding light transmissivity characteristics of the liquid crystal cell.
Referring to FIG. 13, a driving data voltage having a frequency of 60 Hz is uniformly applied to aforementioned FLC cells (e.g., FLC cells fabricated in the presence of an applied electric field having a negative polarity) arranged within the LCD panel. Accordingly, the polarity of the applied driving data voltage is inverted each successive frame period of the LCD panel (i.e., 16.7 ms). As a result, the FLC cells transmit light during odd frame periods 1Fr, 3Fr, 5Fr, etc., when the applied driving data voltage generates an electric field having a positive polarity (e.g., when the applied driving data voltage has a positive polarity, +V), and transmits substantially no light (i.e., intercepts light) when the applied driving data voltage generates an electric field having a negative polarity (e.g., when the applied driving data voltage has a negative polarity, −V). Therefore, when LCD panels including uniformly fabricated Half V-Switching Mode FLC cells are driven, the overall brightness of the LCD panel decreases and pictures displayed by the LCD panel appear to flicker because viewers perceive the transmitted light periodically within each frame period of the LCD panel.
Further, blurring or contour trailing occurs when the LCD panel displays moving pictures due to a slow response time of the FLC material and due to predetermined maintenance characteristics of the FLC material. Cathode ray tubes (CRTs) do not display pictures by maintain data voltages. Rather, CRTs are a type of impulse display system capable of displaying pictures instantaneously. Accordingly, the aforementioned blurring or contour trailing does not occur when moving pictures are displayed by CRTs. Referring to FIG. 14, CRTs display pictures by irradiating electrons onto a portion of a fluorescent screen for short amount of time within each frame period. Accordingly, each portion of the fluorescent screen remains dark for a portion of each frame period. In contrast, and with reference to FIG. 15, LCDs display pictures by charging data voltages to liquid crystal cells during a scanning period when gate high voltages (Vgh) are applied, wherein, once they are charged, the data voltages are maintained within the liquid crystal cells until they are refreshed in a successive frame period.